Normal map not working correctly
your normal map looks quite good no obvious artifacts or anything wrong with it, same goes for your Image Texture node and the rest in the node editor.
imho I would suggest you to do a quick retopo in order to keep (roughly) the basic shape of your sculpted cube (hi-poly). Normal maps are not able to follow curvatures and they cant fake the edges so it is not that strange you get this result. You need a minimum of actual geometry to use them sufficiently !